Bruno Colas

Gene flow and local adaptation in two endemic plant species

Abstract In order to detect the evolutionary potential of two endangered species, Brassica insularis (Brassicaceae) and Centaurea corymbosa (Asteraceae), within and among-population genetic variation for both quantitative traits and allozymic markers was examined. Four populations of each species were studied, representing a large proportion of extant populations. High values of θST (0.213 and 0.364 for B. insularis and C. corymbosa respectively) suggested that low amounts of gene flow occur among the study populations. In each species, the genetic distance based on allozymes (estimated by the ratio (θST/1−θST)) was positively correlated with the geographical distance, indicating isolation by distance. In contrast to previous studies in either outcrossing or selfing plant species, and especially for B. insularis, population differentiation for quantitative traits (QST) was generally found lower than differentiation for allozymes (θST), suggesting that the populations studied were experiencing similar selective forces acting upon the quantitative traits measured. Such forces would be strong enough to counteract local genetic drift. Interestingly, for both species QSTs were statistically independent of geographical distance, in contrast to the marginally significant positive isolation by distance shown by θST. Altogether, these results suggest that θSTs might not always be used as conservative estimates of QSTs, and might instead overestimate the evolutionary potential of endangered species. This would be especially expected in narrow-endemic species, whose ecological niche is often so restricted that indeed homogeneous selective forces are likely to occur, whereas small population sizes and restricted dispersal are likely to produce strong differentiation for neutral variation. In fact, knowledge of both neutral and quantitative diversity patterns allows identification of those traits undergoing natural selection, and could be useful in designing reinforcement or reintroduction programs. However, this approach might have limitations too, in the presence of outbreeding depression due to locally coevolved gene complexes.

SPATIAL AND TEMPORAL DEMOGRAPHIC VARIABILITY IN THE ENDEMIC PLANT SPECIES CENTAUREA CORYMBOSA (ASTERACEAE)

Centaurea corymbosa is an endemic plant species restricted to a 3-km2 area in southern France. This species is known from only six small populations that are highly differentiated genetically. Matrix models based on eight years of data (1994–2001) were used to assess the pattern of variation in the demographic vital rates of this species, and to investigate the causes of their variation. Asymptotic growth rates λ varied widely between years and populations (0.613–1.424). Randomization tests were developed to test for spatial and temporal variation in the asymptotic growth rates. These tests rely on individual data on both survival and fecundity. As our demographic survey only allowed us to estimate average fecundities, additional fecundity data collected from 1994 to 1996 were used to assess the distribution of individual fecundity expected within populations under demographic stochasticity or sampling error. Randomization tests showed that asymptotic growth rates were significantly different between popu...

Fine‐scale genetic structure and gene dispersal in Centaurea corymbosa (Asteraceae) I. Pattern of pollen dispersal

Pollen dispersal was characterized within a population of the narrowly endemic perennial herb, Centaurea corymbosa, using exclusion‐based and likelihood‐based paternity analyses carried out on microsatellite data. Data were used to fit a model of pollen dispersal and to estimate the rates of pollen flow and mutation/genotyping error, by developing a new method. Selfing was rare (1.6%). Pollen dispersed isotropically around each flowering plant following a leptokurtic distribution, with 50% of mating pairs separated by less than 11 m, but 22% by more than 40 m. Estimates of pollen flow lacked precision (0–25%), partially because mutations and/or genotyping errors (0.03–1%) could also explain the occurrence of offspring without a compatible candidate father. However, the pollen pool that fertilized these offspring was little differentiated from the adults of the population whereas strongly differentiated from the other populations, suggesting that pollen flow rate among populations was low. Our results suggest that pollen dispersal is too extended to allow differentiation by local adaptation within a population. However, among populations, gene flow might be low enough for such processes to occur.

Spatio-temporal variation of reproductive success and conservation of the narrow-endemic Centaurea corymbosa (Asteraceae)

The viability of a metapopulation of the rare, cliff-dwelling Centaurea corymbosa depends critically on the persistence of extant local populations, as it has a very low colonising ability. The variation in components of seed production in space and time was investigated and compared with the variation of supposedly causal plant and population traits. Temporal variation in the number of capitula per plant and in the number of ovules per capitulum were related to resource limitation between years and flowering periods, respectively. The seed-to-ovule ratio also varied widely, because of variation in fertilization rate, and, to a lesser extent, in abortion rate; predation being negligible. Isolated individuals had lower fertilization rates than individuals within dense patches. But despite a high density, the smallest studied population showed the lowest fertilization rates and highest abortion rates. This Allee effect on seed production represents a possible threat for small and/or low-density populations. Reinforcing populations of low density and supplying the smallest populations with new genotypes, by seed deposition into clefts of rocks, are suggested as possible conservation management strategies for C. corymbosa.

Variation in dispersal traits in a narrow-endemic plant species, Centaurea corymbosa Pourret. (Asteraceae)

The existence of genetic variability for dispersal is a crucial issue for organisms facing increased habitat fragmentation and climate change. We study the genetic basis and evolutionary potential for diaspore traits related to dispersal in Centaurea corymbosa. Using diaspores collected in natural conditions in four of the six extant populations of this narrow-endemic plant species and diaspores produced in a common garden experiment, we study the variation for pappus and achene sizes, and diaspore mass. Using a sample of achenes from the common garden experiment, we find that the best predictor of terminal velocity is a linear combination of pappus length, achene width, and achene weight. We find significant differences among populations for all traits in both conditions, as well as significant differences among families within population. Although the differences among populations for some traits are not exactly the same in controlled conditions compared to natural conditions, the ranking of populations according to their mean trait values is consistent in both conditions. Our study is therefore one of the first to show a correlation between phenotypic differentiation for dispersal traits in natural conditions vs. controlled conditions. We also show evidence of genetic variation for traits commonly thought to be involved in dispersal ability, suggesting the potential for evolutionary changes following environmental change and management actions.

Population structure of an endangered species living in contrasted habitats: Parnassia palustris (Saxifragaceae)

In endangered species, it is critical to analyse the level at which populations interact (i.e. dispersal) as well as the levels of inbreeding and local adaptation to set up conservation policies. These parameters were investigated in the endangered species Parnassia palustris living in contrasted habitats. We analysed population structure in 14 populations of northern France for isozymes, cpDNA markers and phenotypic traits related to fitness. Within population genetic diversity and inbreeding coefficients were not correlated to population size. Populations seem not to have undergone severe recent bottleneck. Conversely to pollen migration, seed migration seems limited at a regional scale, which could prevent colonization of new sites even if suitable habitats appear. Finally, the habitat type affects neither within‐population genetic diversity nor genetic and phenotypic differentiation among populations. Thus, even if unnoticed local adaptation to habitats exists, it does not influence gene flow between populations.